Digital audio transmission requires a considerable amount of memory and bandwidth. To achieve an efficient transmission, signal compression techniques need to be employed. Efficient coding systems are those that are capable of optimally eliminating irrelevant and redundant parts of an audio stream. For example, the former of the two, is achieved by reducing psycho acoustical irrelevancy through psychoacoustics analysis. As another example, the latter of the two is accomplished by modeling the signal using a set of functions or through a prediction tool.
Generally, there are two conventional coding approaches used for compression purposes. The first is approach is typically transform coding, while the second is approach is typically parametric coding. Conventional transform coders use the frequency domain representations of the signal to perform psychoacoustics analysis and allocate the quantization noise below the noticeable level of human auditory systems. Conventional parametric coders, on the other hand, decompose signals into parameterized components. Accordingly, only these parameters are subsequently coded.
Transform coders typically operate at a much higher bit rates and exhibit higher qualities than conventional parametric coders. Some examples of transform coder are MPEG layer 1 to layer 3, MPEG-AAC etc., all of which require around 128 kbps for a good stereo quality. Parametric coders typically have an operating bit rate below 32 kbps. An example of a typical parametric coder is a MPEG-HILN coder. Some conventional high quality encoding efforts combine the two approaches above and generally result in a “hybrid” coder.
An enhanced AAC plus coder is a conventional example of hybrid coder. Enhanced AAC plus coders typically combine a transform coder (AAC) with parameterized high frequency components (also generally known as Spectral Band Replication) and parametric stereo coder. A set of spatial parameters is firstly extracted from a stereo streams. After which, a stereo to mono downmix is performed, and the mono stream is passed to the core transform coder. In the case of enhanced AAC plus, further parameterization is done to represent the high frequency component of this mono stream, and only the lower half of the mono streams is processed by the core transform coder. MP3 pro uses a similar scheme with MP3 as the core transform coder.
The scheme to represent stereo audio as monaural downmix and a set of spatial parameters which describe the original stereo image is commonly known as Parametric Stereo (PS). FIG. 1 depicts the general structure of a conventional MPEG parametric stereo encoder 100. One frame consisting of 2048 time domain audio samples at both channels is filtered by a 64-band complex-modulated quadrature mirror filter (QMF) followed by down-sampling by a factor of 64. To increase the resolution in the lower frequency region where human ears are most sensitive, further filtering is performed to the first few lower frequency channels to get a total of 71 complex-subband samples. These hybrid filtering results are then grouped non-linearly into 20 stereo bands to follow the equivalent rectangular bandwidth (ERB) with an increasing/coarser bandwidth towards the higher frequency. A set of spatial parameters is extracted from each stereo band and differentially coded into the bit stream. These parameters are IID (Interchannel Intensity Difference), IC (Interchannel Coherence), IPD (Interchannel Phase Difference) and OPD (Overall Phase Difference).
Interchannel intensity difference is defined as the logarithm of the power ratio between the two channels as shown in Equation 1 below.
                              I          ⁢                                          ⁢          I          ⁢                                          ⁢                      D            ⁡                          [              b              ]                                      =                  10          ⁢                                          ⁢                      log            10                    ⁢                                                    ∑                                  n                  =                  0                                                  n                  =                  31                                            ⁢                                                          ⁢                                                ∑                                      k                    =                                          k                      b                                                                            k                                          b                      +                                              1                                                  -                          1                                                                                                                    ⁢                                                                  ⁢                                                      l                    ⁡                                          (                                              k                        ,                        n                                            )                                                        ⁢                                                            l                      *                                        ⁡                                          (                                              k                        ,                        n                                            )                                                                                                                          ∑                                  n                  =                  0                                                  n                  =                  31                                            ⁢                                                          ⁢                                                ∑                                      k                    =                                          k                      b                                                                            k                                          b                      +                                              1                                                  -                          1                                                                                                                    ⁢                                                      r                    ⁡                                          (                                              k                        ,                        n                                            )                                                        ⁢                                                            r                      *                                        ⁡                                          (                                              k                        ,                        n                                            )                                                                                                                              (                  Eqn          .                                          ⁢          1                )            
In Equation 1, l and r are the left and right channel complex subband sample, respectively. In addition, k is the frequency channel index, n is the subband sample index, and b is the stereo band index.
The interchannel coherence is defined as the normalized cross-correlation coefficient after phase alignment according to the IPD as shown in Equation 2 below.
                              I          ⁢                                          ⁢                      C            ⁡                          [              b              ]                                      =                                                                        ∑                                  n                  =                  0                                                  n                  =                  31                                            ⁢                                                ∑                                      k                    =                                          k                      b                                                                            k                                          b                      +                                              1                                                  -                          1                                                                                                                    ⁢                                                                  ⁢                                  l                  ⁢                                      (                                          k                      ,                      n                                        )                                    ⁢                                                            r                      *                                        ⁡                                          (                                              k                        ,                        n                                            )                                                                                                                                                          (                                                      ∑                                          n                      =                      0                                                              n                      =                      31                                                        ⁢                                                            ∑                                              k                        =                                                  k                          b                                                                                            k                                                  b                          +                                                      1                                                          -                              1                                                                                                                                            ⁢                                                                  l                        ⁡                                                  (                                                      k                            ,                            n                                                    )                                                                    ⁢                                                                        l                          *                                                ⁡                                                  (                                                      k                            ,                            n                                                    )                                                                                                                    )                            ⁢                              (                                                      ∑                                          n                      =                      0                                                              n                      =                      31                                                        ⁢                                                            ∑                                              k                        =                                                  k                          b                                                                                            k                                                  b                          +                                                      1                                                          -                              1                                                                                                                                            ⁢                                          r                      ⁢                                                                                          ⁢                                              (                                                  k                          ,                          n                                                )                                            ⁢                                                                        r                          *                                                ⁡                                                  (                                                      k                            ,                            n                                                    )                                                                                                                    )                                                                        (                  Eqn          .                                          ⁢          2                )            
When the phase parameters are not used, the IC alone should represent the phase or time difference between the two channels. In this case, the IC is defined as shown in Equation 3 below.
                              I          ⁢                                          ⁢                      C            ⁡                          [              b              ]                                      =                              Re            ⁢                          {                                                ∑                                      n                    =                    0                                                        n                    =                    31                                                  ⁢                                                                  ⁢                                                      ∑                                          k                      =                                              k                        b                                                                                    k                                              b                        +                                                  1                                                      -                            1                                                                                                                                ⁢                                                                          ⁢                                                            l                      ⁡                                              (                                                  k                          ,                          n                                                )                                                              ⁢                                                                  r                        *                                            ⁡                                              (                                                  k                          ,                          n                                                )                                                                                                        }                                                                          (                                                      ∑                                          n                      =                      0                                                              n                      =                      31                                                        ⁢                                                                          ⁢                                                            ∑                                              k                        =                                                  k                          b                                                                                            k                                                  b                          +                                                      1                                                          -                              1                                                                                                                                            ⁢                                                                                  ⁢                                                                  l                        ⁡                                                  (                                                      k                            ,                            n                                                    )                                                                    ⁢                                                                        l                          *                                                ⁡                                                  (                                                      k                            ,                            n                                                    )                                                                                                                    )                            ⁢                              (                                                      ∑                                          n                      =                      0                                                              n                      =                      31                                                        ⁢                                                                          ⁢                                                            ∑                                              k                        =                                                  k                          b                                                                                            k                                                  b                          +                                                      1                                                          -                              1                                                                                                                                            ⁢                                                                                  ⁢                                                                  r                        ⁡                                                  (                                                      k                            ,                            n                                                    )                                                                    ⁢                                                                        r                          *                                                ⁡                                                  (                                                      k                            ,                            n                                                    )                                                                                                                    )                                                                        (                  Eqn          .                                          ⁢          3                )            
IPD and OPD are the phase difference between the two channels and between the left and the mono downmix, respectively, as shown in Equations 4 and 5 below.
                              IPD          [          b          〛                =                  ∠          ⁡                      (                                          ∑                                  n                  =                  0                                                  n                  =                  31                                            ⁢                                                          ⁢                                                ∑                                      k                    =                                          k                      b                                                                                                  k                                              b                        +                        1                                                              -                    1                                                  ⁢                                                                  ⁢                                                      l                    ⁡                                          (                                              k                        ,                        n                                            )                                                        ⁢                                                            r                      *                                        ⁡                                          (                                              k                        ,                        n                                            )                                                                                            )                                              (                  Eqn          .                                          ⁢          4                )                                          OPD          [          b          〛                =                  ∠          ⁡                      (                                          ∑                                  n                  =                  0                                                  n                  =                  31                                            ⁢                                                          ⁢                                                ∑                                      k                    =                                          k                      b                                                                                                  k                                              b                        +                        1                                                              -                    1                                                  ⁢                                                      l                    ⁡                                          (                                              k                        ,                        n                                            )                                                        ⁢                                                            m                      *                                        ⁡                                          (                                              k                        ,                        n                                            )                                                                                            )                                              (                  Eqn          .                                          ⁢          5                )            
The mono downmix stream m(k,n) is defined as a linear combination of the left and right channel as shown in Equation 6.m(k,n)=w1l(k,n)+w2r(k,n)  (Eqn. 6)
In Equation 6, w1 and w2 are the weights to determine the content of each of the channel in the mono downmix signal. Generally, w1 and w2 are set to 0.5 to have an output that is the average of the two channels. However, this scheme bears the risk that the power of the downmix signal strongly depends on the cross correlation of the two input signals. The resulting monaural signal can be further processed or synthesized back into time domain and passed to a conventional mono audio coder.
There is therefore a need for a method and system of providing an alternative low power implementation of a hybrid encoder, for example, in the parametric stereo encoder portion.